1. Field of the Invention
This invention generally relates to the extraction of lignin, especially demethylated lignin, from lignin-containing aqueous solutions such as, for example, from the raffinate which is a by-product of dimethyl sulfide production which is, in turn, a by-product of wood pulping operations. This invention also relates to the use of demethylated lignin in the preparation of resol-type resins such as those employed in the manufacture of wood adhesives used to make structural wood products such as, for example, plywood and particleboard, wood veneers and the like.
2. Description of the Prior Art
Adhesive resins such as those used in the manufacture of structural wood products are either aminoresins or phenolic resins. Aminoresins are polymeric products of the reaction of an aldehyde with compounds containing an amino group, particularly urea and melamine. In virtually all aminoresins the aldehyde component is formaldehyde and by far the dominant aminoresin is urea-formaldehyde because of its relatively low cost compared to other resins used in wood adhesives. The major disadvantage of aminoresins, especially urea-formaldehyde resin, is that they are not totally water resistant, and consequently their gluelines will eventually delaminate. They also are known to release formaldehyde during their slow water hydrolysis.
Phenolic resins are polymeric products of the reaction of an aldehyde with compounds containing a phenolic hydroxyl group. The phenolic component is most often phenol, but may be cresol, resorcinol, or catechol, or the like. The rate of reaction is highly dependent on the relative reactivity of the phenolic substance used. Resorcinol is much more reactive than phenol and consequently is used in "cold-setting" resin adhesives. Catechol is also more reactive than phenol, but somewhat less reactive than resorcinol. The phenolic component used in the manufacture of such resins may also be a mixture of phenolic substances, such as phenol-resorcinol resins. Formaldehyde is the most common aldehyde component although others such as furfural are occasionally used. Phenol and the other phenolic substances are considerably more expensive than urea; however, phenolic resins give weather-and boil-proof gluelines. Moreover, phenolic resins do not release formaldehyde.
The reaction between a phenolic substance and an aldehyde can be either acid catalyzed or base catalyzed. Under acidic conditions and with an excess molar ratio of phenolic component an essentially linear low-degree-of-polymerization "prepolymer" is formed. This material is known as a novolak. Polymerization can be continued to an infusible solid through the addition of sufficient aldehyde. The completely polymerized material, often referred to as cured or hardened, is usually used in molded products. Novolak prepolymers are usually not water soluble; however, under sufficiently basic conditions the novolak may form a salt and become water soluble. Under basic conditions aldehyde can be added to the mixture and a base catalyzed polymerization performed at elevated temperatures.
Under basic conditions and with an excess molar ratio of aldehyde a highly branched low-molecular-weight "prepolymer" resin known as a resol is formed. This material is usually water soluble because of salt formation in the basic solution. No additional aldehyde is required for continued polymerization to an infusible water insoluble product. The polymerization is usually continued by elevating the temperature of the reaction. Resol prepolymers are the phenolic resins usually used for wood adhesives. They are preferred over novolaks for several reasons: (1) The resol adhesive is a single component entity in that no additional aldehyde is required. (2) Their continued polymerization to achieve a highly water-resistant glueline is easily controlled by elevating the temperature. (3) The basic adhesive mixture does not cause long term wood degradation adjacent to the glueline as is the case with acid catalyzed material.
By far, the most common phenolic-type resin for wood adhesives is phenol-formaldehyde resol resin. Resorcinol-formaldehyde cold-setting adhesives are of the novolak-type, but without an acidic catalyst due to their high reactivity. These cold-setting adhesives are two component systems requiring aldehyde addition to the novolak prepolymer immediately before use; otherwise there is no means of controlling the polymerization to an infusible product.
Phenol, as well as the other phenolic substances noted above are relatively expensive. Phenol is also petroleum derived, i.e., it is a petrochemical. In general, it would be regarded as highly advantageous to utilize a less expensive, nonpetrochemical phenolic material in such resins if this were possible. For example, a biomass derived phenolic material which is readily available to the forest products industry, and which is presently under-utilized, would be an ideal substitute for petrochemical derived phenol. Such a substitution would imply that the industry using the wood adhesives also would control the raw material required in the preparation of the resin used in these adhesives.
Lignin is the dominant phenolic substance in biomass and it is grossly under-utilized. Most lignin, especially that which is produced as a by-product of the pulping industry, is used primarily as an energy source. It has long been recognized that this is a low value use of this lignin source; consequently many technical proposals have been made for upgrading the use of this material as well for obtaining an economic advantage over the petroleum derived phenol used for this purpose. The allure of using a biomass lignin resource in wood adhesives has produced patents and other publications which describe the use of lignin in resins used for wood adhesives. The majority of these publications describe the use of various lignin sulfonates, by-products of the sulfite pulping industry, although many other types of lignin are also mentioned. This information is comprehensively covered by H.H. Nimz in "Lignin-Based Wood Adhesives", a chapter in "Wood Adhesives: Chemistry and Technology" edited by A. Pizzi, Marcel Dekker, Inc. publisher (1983).
There are other references made in the literature which more particularly address the use of demethylated lignin in resins for adhesives. Since the methods of this patent disclosure also involve the use of demethylated lignin, a critical review emphasizing how the methods of this patent disclosure differ from what is believed to be the most pertinent of these references is in order. Such a review should begin by first noting that kraft lignin is known to be slightly demethylated during the pulping process. That is to say that, by way of example, the methoxyl content of softwood kraft lignin is 12.6%, whereas that of the original lignin in the original softwood wood is 14.5%. For the purposes of this patent disclosure the term "demethylated lignin", regardless of its source, should be taken to mean lignin which has undergone some substantial degree of demethylation, e.g., such that its methoxy content is from about 11% to essentially 0% by weight of the original lignin material. For purposes of this patent application, the term demethylated lignin should also be taken to mean other lignin derived materials such as, for example, any pulping by-product lignin regardless of process, and by-product lignin from biomass processing which also have been subjected to some degree of demethylation.
Regardless of terminology, it is well known that only slight degrees of demethylation are insufficient to adequately improve the resin forming properties of kraft lignin; consequently many attempts have been made to modify and then utilize kraft lignin, as well as other lignin-containing sources, as substitutes for petroleum derived phenol in the manufacture of wood adhesives.
For example, the publication "Thermosetting Resins of Demethylated and Tall Oil Lignins" in Khim.Ispol'z Lignina 1974:428-433 teaches a process wherein demethylated lignin, which was a product of an alkaline hydrolysis of kraft lignin, is employed. However, it has been noted that an alkali, even at high temperatures, is a poor demethylating agent. Hence, little demethylation can be accomplished by this process as was indicated by its need for petrochemical derived phenol to produce its resin. Moreover, the temperature required for hardening in this process is also high, compared to even a typical phenol-formaldehyde resol resin. This indicates that deactivation rather than activation takes place; this would not occur with substantial demethylation.
A publication titled "Kraft Lignin Utilization in Adhesives" found in Wood Science Technology 22:157-165 (1988), evaluated demethylated lignin in several resins for adhesives. However, since kraft lignin is insoluble in acid solutions such as those of the disclosed acidic chromate solutions, the reaction must be performed in a heterogeneous two-phase system wherein only limited demethylation takes place. In any event, the publication concluded that "Resins prepared with demethylated lignin are not included [in the estimated cost evaluations] due to their deficient behavior in particleboard panels."
A publication by Gapta and Sehgal in Holzforschung and Holzverwertung (1978) 30:85-87 discloses demethylation by hydriodic acid treatment of kraft lignin. However, because of numerous technical difficulties, use of their demethylated lignin led to only about a 10% reduction in the phenol requirement in the adhesive. The difficulties generally involve the fact that kraft lignin is insoluble in an acidic reaction medium, isolation of the demethylated lignin by precipitation and premature gelling of the demethylated lignin.
An article by Enkvist et al, published in Tappi 45, 128(1962) suggests the possibility of using demethylated lignins to produce condensation polymers with formaldehyde. The material employed in this process was degradation products of a demethylation process used to produce simple compounds such as catechol and related substances. The "demethylated lignin", which was only a minor amount of the total obtainable material, was evaluated for its novolak properties, but not as a resol resin. Moreover, since the isolated demethylated lignin of this reference is somewhat insoluble under acidic conditions it would be rather difficult to prepare a novolak with acceptable properties. It should also be noted that novolaks are not used in adhesives for structural wood products.
In analyzing the problems encountered by the prior art in seeking to employ lignin in place of phenol in the production of wood adhesives, it should also be noted that another difficulty with using such lignin as the phenolic component in resin for wood adhesives is that lignin is less reactive than phenol itself. This problem of lower reactivity when using lignin in place of phenol is apparently the result of two separate causes. The first is that most of the necessary bonding sites for a phenolic-aldehyde-type resin are not available in lignin due to the very nature of the material. The second problem derives from the fact that when lignin is isolated by prior art methods such as those noted above, only a fraction of the original material is actually isolated. Usually the lower molecular weight component is not isolated but instead is lost during the recovery step. Moreover, as a consequence of this reduced reactivity, compared to phenol, a resol prepolymer prepared using lignin requires a longer cure time at elevated temperature and the resulting glueline does not exhibit the same strength properties as a phenol-formaldehyde resol. These conditions also are considered highly unacceptable to the wood products industry. Consequently there are no processes which are widely acceptable to the industry which enable lignin to be substituted for phenol in the manufacture of resol resins.